1. Field of the Invention
The invention generally relates to a laser device and system method used to perform various surgical procedures, and more specifically to a medical laser delivery device and system employing at least one graded index optical fiber and means of altering laser beam's output characteristics.
2. Information Disclosure Statement
Medical laser delivery devices and systems are becoming increasingly more important for effectively performing procedures such as tissue cutting, coagulating, welding, and destroying kidney stones. Traditionally, these systems consist of a laser source, input optics, one or more flexible optical fibers, and an output end or handpiece. For example, U.S. Pat. No. 4,421,382 to Doi et al. and U.S. Pat. No. 5,102,410 to Dressel disclose laser delivery systems and their respective components. These laser systems and devices may also incorporate accessory devices such as cooling means, positioning means, vision fibers, or sensing fibers. The configuration of a particular laser delivery system depends upon the specific surgical application.
Perhaps the most important parameter to consider when configuring a laser delivery system is the interaction of the laser beam with the tissue. This interaction is largely a function of the laser's power density at the target. For example, cutting and coagulation require different power densities. Power density also controls the burn-off of residues from the output end of delivery system. Presently, the prior art employs step index fiber and special tips to control the output power density.
In a step index fiber, the laser beam becomes dispersed, losing its focal points as it propagates down the fiber. Because the laser beam lacks focal points, its density becomes redistributed across the whole diameter of the fiber's core. The power density thus becomes a function of the core's cross-sectional area. Consequently, to increase power density, the diameter of the fiber must be decreased. As the cross section area becomes smaller, however, it becomes more difficult to project the laser beam into the fiber. Thus, the prior art's control over power density becomes limited by the fiber's core size. Prior art devices face this limitation.
This limitation of prior art devices requires the attachment of sculptured sapphire tips to the end of the delivery device as described in U.S. Pat. No. 4,693,244 to Daikuzono. These tips focus the laser to a specific power density depending upon the specific surgical or medical application. Although effective in altering power density, changing tips during the operation can be difficult. For example, in the same operation, an operator may initially use the laser system to make an incision, and later use it to coagulate. This requires changing the sapphire tips at the end of the delivery fiber. Often, however, the output end may not be readily accessible, especially if it lies within the patient or if its sterility might be jeopardized. Therefore, the prior art is limited by its need to access the output end of the delivery system to alter power density. Therefore, a need arises to both control and monitor output power density at the input of the fiber.
Tanaka (U.S. Pat. No. 4,576,160) discloses a method to vary the spot size (and thus the power density) by a two step approach namely by changing the fiber used for transmission to one of the appropriate diameter as a coarse regulation and employing variable optics at the output end for fine regulations. It is thus clear that this prior art cannot overcome the inherent limitations in the transmission power density resulting from the use of the step index fiber and that it further suffers from the need to change the transmission fiber used altogether or to insert a number of fibers into the patient to be endoscopically treated, thus drastically increasing the channel diameter of the incision required. The variable optics on the output end are also difficult to practice in an endoscopic situation: its total space requirement may be prohibitively large and the control means (such as wires) to alter its characteristics during the coarse of a medical procedure without removing the device from the body would further add to the overall diameter of the device.
The laser beam's dispersion in a step index fiber not only limits the prior art's ability to control output power density, but also limits its ability to monitor the same for feedback purposes. To monitor output, the prior art requires monitoring the output power density at the output end of the delivery fiber. For example, the U.S. Pat. No. 4,693,244 and 5,050,597 both to Daikuzono require thermal sensing means, such as a thermocouple, located at the output end to provide feedback. Although U.S. Pat. No. 5,057,099 to Rink monitors infra-red radiation at the input end, Rink's invention is intended to monitor the temperature of the delivery device to insure that its maximum operating temperature is not exceeded. The Rink invention can only control the average over the cross section and over the whole fiber tip; it is unable to pinpoint local hot spots on the fiber end resulting, for instance, from small particles attached or burnt in. It does not monitor the output power density. Indeed, the prior art's use of a step index fiber prevents monitoring local power density from the input end of the fiber.